The present invention relates to image sensing devices and, more particularly, to image sensing devices that comprise an array of thin-film semiconductor photodiodes.
In the development of high-speed, high-resolution facsimile equipment, it has become necessary to provide image sensors comprising large, high density arrays of photodetectors which are capable of fast response and high sensitivity to small changes in light intensity. A typical scanning arrangement used in facsimile equipment is illustrated in FIG. 1. With reference to FIG. 1, a manuscript 1 moves in a transverse direction relative to a linear image sensor 2 comprising a linear array of regularly spaced photodetectors 3, which extend across the width of the manuscript 1. The image of one line of the manuscript at a time is focused onto the photodetector array 3 by a Selfoclens 5. The manuscript is illuminated by two linear arrays of light emitting diodes 4 situated on each side of the Selfoclens 5.
In order to meet the requirements of highspeed, high-resolution facsimile equipment currently being developed, it is generally desirable for the photodetectors of the linear array to have a pitch of approximately 8 per mm or 16 per mm, depending upon the expected size of the patterns in the manuscript to be scanned. The active area of each photodetector should be approximately 100 .mu.m .times. 100 .mu.m for the larger pitch and 50 .mu.m .times. 50 .mu.m for the smaller pitch. Furthermore, the response time of the photodetectors should be such that an A4 or B4 size manuscript can be scanned at the rate of one line in 4 msec or less.
Formerly, image sensors for facsimile equipment have been constructed with linear arrays of charge-coupled devices (CCD's); however, recently there have been developed image sensors constructed with linear arrays of thin-film semiconductor p-i-n photodiodes. The latter construction provides the advantages of allowing larger arrays to be fabricated with improved performance and lower manufacturing cost.
A known structure for a thin-film, p-i-n photodiode array image sensor is illustrated in FIGS. 2 and 3. Referring to FIGS. 2 and 3, the image sensor 20 comprises a plurality of regularly spaced p-i-n photodiodes 51 disposed in a row. The photodiodes 51 are fabricated by first forming a layer 2 of a transparent conducting material, such as indium tin oxide or SnO.sub.2, on a major surface of a glass substrate 1. The transparent conducting layer 2, which has a thickness in the range of several hundred to several thousand angstroms, may be formed by conventional electron beam evaporation, sputtering or chemical vapor deposition (CVD) techniques. Once formed, the layer is patterned into a wide strip by conventional photolithographic and etching techniques.
Following the formation of transparent conducting layer region 2, an amorphous silicon layer 3 (shown after patterning) of approximately 1 .mu.m in thickness is formed by glow discharge decomposition of SiH.sub.4 gas at a relatively low temperature in a reaction chamber containing the substrate 1. During the formation of the amorphous silicon layer 3, appropriate impurities, such as diborane and phosphine, are introduced into the reactor chamber to produce a layer-like p-type impurity zone 31 of approximately 100 .ANG. in thickness adjacent to the transparent conducting layer 2 and a layer-like n-type impurity zone 32 of approximately 500 .ANG. in thickness adjacent the upper surface of the amorphous silicon layer 3. The region of the amorphous silicon layer between the p-type and n-type impurity zones 31 and 32 is undoped (i.e., intrinsic). In some instances, the layer-like p-type impurity zone 31 is formed by depositing a layer of amorphous SiC:H over the transparent conducting layer before the deposition of the amorphous silicon layer.
After deposition of the amorphous silicon layer, a layer of an appropriate metal 4 (shown after patterning), such as aluminum, is deposited by conventional electron beam evaporation to a thickness of approximately 1 .mu.m covering the amorphous silicon layer. The metal layer is then patterned by conventional photolithographic and etching techniques to form square electrode regions 41 and strip-like connecting regions 42. Thereafter, the amorphous silicon layer is subjected to an anisotropic etch using a plasma generated in CF.sub.4 with relatively low frequency RF fields at a relatively low pressure. Owing to the high selectivity of such an etch, the patterned aluminum layer 4 is used as the etch mask to form the separate amorphous polysilicon layers 3 having the same shape as the aluminum layers 4.
The image sensor 20 fabricated in the foregoing manner comprises a linear array of p-i-n photodiodes 51 having a transparent conductor 2 that serves as a common anode electrode, aluminum electrode regions 41 that serve as separate cathode electrodes for the individual photodiodes and amorphous silicon layers 3 between the anode and cathode electrode regions 2 and 41. Each of the amorphous silicon layers 3 has a p-type zone 31 in ohmic contact with the anode electrode 2, an n-type zone making ohmic contact with a respective one of the cathode electrodes 41 and an intrinsic zone between the p-type and n-type zones 31 and 32. During operation, the photodiodes 51 are appropriately reverse biased and light is incident on the amorphous silicon regions of each photodiode through the glass substrate and the transparent common anode electrode 2. Photodetection signals produced by the photodiodes 51 are provided through respective ones of the strip-like aluminum connecting regions 42, which serve to conduct such signals to signal processing circuitry (not shown).
Although the above-described known image sensor structure has the advantages of high performance, high packing density and requiring relatively few processing steps for its fabrication, it has the problem of low manufacturing yield, owing to the amorphous silicon region 3 having exposed side surfaces 8 at the edges thereof which extend between the anode and cathode electrodes 2 and 41. During processing of the photodiode array 20, such exposed side surfaces 8 are subject to being contaminated with aluminum or aluminum-silicide, which tends to create leakage paths between the anode and cathode electrodes 2 and 4 of the photodiodes 51. If the leakage current of any one of the photodiodes 51 of the sensor 20 exceeds a specified maximum value, the entire sensor must be rejected and the manufacturing yield of the sensor is thus reduced.
Accordingly, a need exists for an improved thin-film p-i-n photodiode array image sensor structure which avoids the above-described leakage-current yield loss mechanism that exists in the prior art structure.